Prostate cancer is the largest cause of male cancer death in the United States; many prostate cancer patients have metastatic disease at diagnosis. Vitamin D deficiency is a risk factor for prostate cancer, and 1,25-dihydroxyvitarhin D3 (1,250, the hormonally active form of vitamin D) and noncalcemic analogues have shown therapeutic benefits in clinical trials with prostate cancer patients. Prostate cancer cells produce many growth factors, including parathyroid hormone-related protein (formerly peptide, PTHrP). We show that PTHrP overexpression increases prostate cancer cell (a) proliferation; (b) adhesion to the extracellular matrix proteins collagen types I and IV, laminin, and fibronectin; (c) migration onto laminin and collagen type I; (d) invasion of Matrigel, a reconstituted basement membrane; (e) anchorage-independent cell growth; (f) cell surface expression of the pro-invasive integrin a6|34; and (g) xenograft growth in vivo. 1,250 exerts opposite effects on all these parameters, and downregulates PTHrP gene expression. Both PTHrP and integrin 04 expression are required for the anti-proliferative and anti-invasive effects of 1,250, as shown using siRNAs. We thus hypothesize that (a) PTHrP facilitates prostate cancer metastasis by upregulating pro-invasive integrin expression, and (b) the therapeutic benefits of vitamin D are mediated via both downregulatipn of PTHrP gene expression and a direct effect on integrin expression. This project seeks to understand the relationship between PTHrP, vitamin D and integrin a6p4 in prostate cancer cells, by pursuing three Aims. (1) The pathways via which PTHrP and vitamin D regulate integrin a634 expression will be investigated using siRNAs and integrin promoter deletions/transient transfection assays. Since integrin a6(34 increases cancer aggressiveness, we will determine by immunoprecipitation whether vitamin D alters the association of a6 from (34 to 31. (2) The role of PTHrP and integrin (34 in prostate cancer cell growth and bone metastasis, and the effect of concomitant vitamin D treatment, will be studied in vivo using a nude mouse model. (3) The vitamin D response elements via which vitamin D regulates PTHrP gene expression will be identified by PTHrP gene promoter deletions and transient transfection assays. Understanding the relationship between PTHrP, vitamin D, and pro-invasive integrin expression, and the molecular pathways involved, should lead to the development of new combinatorial therapeutic approaches to manage prostate cancer.